Poison-laced drinking water killed some of the world’s oldest mummies, which are found in the harsh northern deserts of Chile, a new study says.

Arsenic, which occurs in high levels in drinking water in Chile’s northern Camarones Valley, the deadly element likely poisoned the coastal Chinchorro people for centuries, starting at least 7,000 years ago, mummy-hair analyses show.

“I believe [these] ancient people were continuously exposed to arsenic by drinking contaminated water with high arsenic levels [that is] endemic to the Camarones region,” said study leader Bernardo Arriaza of Chile’s Universidad de Tarapacá de Arica.

The Chinchorro, who wouldn’t have been aware they were ingesting the tasteless and invisible toxin, may have suffered from skin, lung, bladder, and kidney cancers, among other serious effects of long-term arsenic exposure.

After they died, the poison victims were sometimes mummified, their internal organs removed and replaced by soil and reeds. Some 7,000 years ago the Chinchorro became the first society known to practice mummification, which transcended social class and included adults, children, and even fetuses.

Arriaza hypothesizes that fetuses and newborns were especially susceptible to arsenic. Because the earliest mummies are fetuses and infants, he suggests that high miscarriage rates may have inspired the mummification process.

But the threat’s not just in the past: The valley’s drinking water still contains a hundred times more arsenic than is considered safe by the World Health Organization. The contamination has forced area residents to get their water elsewhere, Arriaza said.